For each plant species, there exists a wide discrepancy in plant growth due to environmental conditions. Under most conditions, the maximum growth potential of a plant is not realized. Plant breeding has demonstrated that a plant's resources can be redirected to individual organs to enhance growth.
Genetic engineering of plants, which entails the isolation and manipulation of genetic material, e.g., DNA or RNA, and the subsequent introduction of that material into a plant or plant cells, has changed plant breeding and agriculture considerably over recent years. Increased crop food values, higher yields, feed value, reduced production costs, pest resistance, stress tolerance, drought resistance, the production of pharmaceuticals, chemicals and biological molecules as well as other beneficial traits are all potentially achievable through genetic engineering techniques.
Plant growth responds to the increased availability of mineral nutrients in the soil, but shoot and root growth respond differently. Moreover, a direct relationship between mineral nutrient availability and change of growth rate is rarely observed over a larger concentration range. This suggest that plant growth is limited materially by nutrients required for cell growth as well as by signaling pathways that control the rate of organ growth for the overall benefit of the plant. Although the components of these regulatory pathways have not been identified, they define two distinct avenues to potentially improve plant growth.
Plants rarely grow under optimal conditions. Plant growth can be limited by water availability, mineral nutrients and a short growing season. Drought tolerance in genetic variants of a given species is well correlated with the penetration depth of its root system into the soil. Fertilizers are often not optimally utilized because of insufficiently penetrating root systems. Although the induction of flowering can now be controlled, thereby extending the potential growth range of some important crop species, this does not in itself lead to increased biomass.
The ability to manipulate gene expression provides a means of producing new characteristics in transformed plants. For example, the ability to increase the size of a plant's root system would permit increased nutrient assimilation from the soil. Moreover, the ability to increase leaf growth would increase the capacity of a plant to assimilate solar energy. Obviously, the ability to control the growth of an entire plant, or specific target organs thereof would be very desirable.
SOB1/OBP3 is a member of the Dof family of transcription factors that, to date, remain unique to plants. Computer programs that predict gene function have identified more than 50 Dof transcription factors in the Arabidopsis genome (Riechmann, J. L. and Ratcliffe, O. J. (2000) Curr Opin Plant Bio 3:423-434). This family of transcriptional regulators shares a conserved DNA-binding domain made up of 52 amino acid residues in which a CX2CX21CX2C motif is predicted to form a single zinc finger, hence their name domain of one finger. These transcription factors are thought to have a common core recognition sequence of AAAG (Yanagisawa, S. and Schmidt, R. J. (1999) Plant J 17:209-214).
Transcriptional activation, DNA binding and mRNA accumulation studies have been previously reported for OBP3 and two other OBPs from Arabidopsis (Kang, H.-G. and Singh, K. B. (2000) Plant J 21:329-339). Though these studies show salicylic acid and auxin induction of all three OBP mRNAs, they have not revealed a role for these transcription factors during development. The experimental findings of Kang and Singh (2000) demonstrated that constitutive expression of the OBP3 gene in Arabidopsis results in severely dwarfed plants with stunted root growth. Applicants speculate that the differences in phenotypes between 35S:OBP3 (the nucleic acid construct utilized by Kang and Singh), and sob1-D phyB-4 (the nucleic acid construct of this invention) may be caused by the differences between constitutive expression seen with the CaMV 35S promoter and the amplified transcriptional expression patterns often seen in activation-tagging mutants (Neff, M. M. et al. (1999) Proc Natl Acad Sci USA 96:15316-15323; Weigel, D. et al. (2000) Plant Physiol 122:1003-1013).
There is a need for a mechanism to genetically alter plants to efficiently and effectively control their size and stature with completely normal and healthy root growth. The present invention provides genetically stable plants that are either larger or smaller based on the over- or under-expression of the SOB1/OBP3 gene. Applicants further provide plant cells transformed by OBP3 to control photomorphogenesis.